[codex] Require modern bash for KDA launches

[BBuf]

Summary

• Add a KernelPilot launcher guard that selects a modern Bash for KDA launches and rejects macOS /bin/bash 3.2.
• Forward the selected Bash through PATH, SHELL, and KDA_BASH_BIN so spawned Claude/Humanize hooks resolve #!/usr/bin/env bash to the safe runtime.
• Document KDA_BASH_BIN in the README and Ghostty/Claude workflow guide, and add the same requirement to generated Humanize draft constraints.

Why

macOS still ships /bin/bash 3.2 even when Homebrew Bash is installed. Humanize hook paths that run under Bash 3.2 can hit set -u empty-array expansion failures. KernelPilot now fails early or re-execs into the selected modern Bash before creating task state.

Validation

• bash -n scripts/launch_kda_kernel_task.sh scripts/launch_kernels/*.sh
• KDA_BASH_BIN=/bin/bash KDA_NO_CLAUDE=1 scripts/launch_kda_kernel_task.sh kernels/b200_diffusion_group_norm_silu__multi_shape exits 127 with the expected modern-Bash error.